Pressure vessels are typically subjected to cyclic thermal and mechanical stresses due to changes in internal fluid pressure and temperature. These cyclic stresses can limit the number and/or magnitude of pressure and/or temperature cycles that the pressure vessels can withstand. Historically, pressure vessels have bores, or penetrations extending through the shell of the pressure vessel. Conduits such as pipes are attached to the pressure vessel such that the penetration and the pipes are in fluid communication with one another to allow for the ingress and egress of fluids to and from the pressure vessel. Stress concentrations exist at the intersection of the pipe(s) and the shell of the pressure vessel. These stress concentrations result in higher stresses and often become a limiting factor in the design of the pressure vessel for phenomena such as fatigue and/or cracking of the magnetite layer that can form on the metal surface, and which may limit the useful lifetime.
Such a pressure vessel may be a boiler or steam drum of an evaporator system as shown in FIG. 1. Referring to FIG. 1, an exemplary prior art evaporator system 100 of a heat recovery steam generator is depicted that comprises an evaporator 102 and a steam drum 104. The steam drum 104 is in fluid communication with the evaporator 102. In a natural circulation heat recovery steam generator, either no flow or minimal flow is established until boiling begins in the evaporator 102. This generally results in a very rapid rise in the steam drum 104 temperature.
For example, for a cold start the water temperature inside the steam drum 104 can rise from 15° C. to 100° C. in less than 10 minutes. This produces a large thermal gradient and hence compressive stress in the steam drum 104 wall. As the pressure in the steam drum 104 increases, the temperature gradient through the drum wall is reduced and consequently the stress due to pressure becomes the dominant stress in the drum. The stress due to pressure (with increased pressure in the steam drum 104) is a tensile stress. The stress range for the drum is determined by the difference between the final tensile stress at full load (pressure) and the initial compressive thermal stress. Boiler Design Codes (such as ASME and EN) impose limits on the stress at design pressure. Some codes, such as for example EN12952-3, also include limits on the permissible stress range for a startup-shutdown cycle. These limits are intended to protect against fatigue damage and phenomena such as cracking of the magnetite layer that forms on the surface of the steel at operating temperature.
Furthermore, steam boilers are provided with a means of determining the water level in the steam drum, as shown in FIG. 2. Water level is typically measured by means of a sight glass and/or pressure transducers 106, which are connected to the drum 104 by an upper and lower connecting tube (nozzle) 108. Boiler Design Code EN 12952-7:2002(E) Section 5.4.2 states “The connecting tubes between the steam boiler and the local water level indicators shall have an inside diameter of at least 20 mm. If the water level indicators are connected by means of common connecting lines or if the water side connecting tubes are longer than 750 mm, the latter shall have an inside diameter of at least 40 mm. Connecting tubes on the steam side shall be designed so that condensate does not accumulate. Water-side connection tubes shall always be arranged horizontally to the water level indicators.” This requirement means that the connecting tubes 108 would typically penetrate the boiler drum non-radially as shown in FIG. 2. The non-radial arrangement results in a high stress concentration as shown in FIG. 3.
Referring to FIG. 3, the results of a finite element analysis, in the form of a stress contour plot of a cut-away view of a portion of a nozzle assembly 109, are shown. The stress contour plot depicts areas of varying stress, the stress contours being superimposed over a section of a known prior art nozzle assembly. The nozzle assembly 109 includes a nozzle that extends through an aperture to an interior area defined by a pressure vessel wall 104. In the illustrated embodiment the area of maximum local stress is located at the intersection at 110 defined between the nozzle 109 and an interior surface of the pressure vessel wall. In general, the nozzle 109 is attached to the pressure vessel 104 via welding. This stress concentration can result in a stress range of greater than 600 megapascals (MPa) in the high pressure drums at 110 during cold startups of Heat Recovery Steam Generators (HRSG), for example, that operate in the range of 150 bar or higher. EN 12952-3 section 13.4.3 requires that the stress range be less than 600 MPa to avoid magnetite cracking. The combination of these requirements make it difficult for HRSG high pressure drums with standard connecting tube arrangements to meet the requirements of the EN Boiler Design Code.
A new approach is suggested by the present invention in which a radial nozzle assembly is used in place of the horizontal connecting nozzle, the radial nozzle assembly being large enough so that a continuous horizontal path is maintained from the inside of the drum to a sensing line 242 as shown in FIG. 5. This configuration results in reduced stress concentrations and lowers the stress range to below 600 MPa as shown in FIG. 7.